1. Field of the Invention
The present invention relates to a vehicle motion control system for controlling vehicle motion, and more particularly to the control system which performs an anti-skid control to prevent a wheel from being locked during braking, by controlling the braking force applied to the wheel, and which also performs a steering control by braking to restrain an excessive oversteer and excessive understeer which will occur during, for example, cornering, by applying a braking force to each wheel of the vehicle irrespective of depression of a brake pedal.
2. Description of the Related Arts
According to a prior anti-skid control system, if a jumping or turning motion of a vehicle occurred during braking, an anti-skid control would be initiated earlier than the control should start in accordance with locking condition of a wheel, as described in the U.S. Pat. No. 5,505,532, for example. In order to prevent the anti-skid control from being initiated by an instantaneous decrease of load which occurs when the vehicle jumps or turns during braking, the publication proposes an anti-skid control system which measures the load applied to the vehicle, and provides a slower response for starting the anti-skid control as the load becomes lower.
Recently, a vehicle motion control system is proposed for controlling the braking force applied to each wheel of the vehicle irrespective of depression of a brake pedal to maintain vehicle stability. According to the vehicle motion control system, if the vehicle is braked in response to depression of the brake pedal during the vehicle is running on a stepped road or a rough road, for example, the anti-skid control might be initiated earlier than the control should start, as described above. In order to avoid this, when the vehicle is running on the stepped road or rough road, a relatively slow response is provided for starting the anti-skid control, in general, as described in the publication.
According to the system as disclosed in the publication, however, the determination of the stepped or rough road is made on the basis of the load applied to the vehicle, so that it is not easy to avoid a delay in starting the anti-skid operation when the vehicle is running on a normal road. To avoid this delay, if a relatively fast response is provided for starting the anti-skid control, the control will be initiated immediately when the vehicle enters onto the stepped or rough road. As a result, it is difficult to solve the above-described problem by changing a condition for starting the anti-skid control, and the same is true of various vehicle motion control modes such as the steering control by braking including an oversteer restraining control and an understeer restraining control, a front-rear braking force distribution control and the like.